30 LIFE AND HER CHILDREN. 
shells into which they can draw back entirely if they 
are attacked, but the flint-builders seem very careless 
in this respect, for they have large holes all over their 
flinty skeletons. But then, on the other hand, notice 
Fig. S. 
Flint Shells built by slime animals.* 
Immensely magnified : the real sizes are from a mere speck to that of 
one of these letters. 
how they send out sharp spikes, which must be un- 
comfortable for any animal trying to snap at them, 
although as we have seen (p. 16) the soft thread- 
* Radiolariae or Polycistince. a, Petalospyris ; b, Ethmosphsera j 
c, Diploconus ; </, Dictyopodium ; t, Heliosphaera ; f, Actinomma. 
